This invention relates generally to a system and method for measuring properties of a semiconductor substrate. In particular, the invention relates to a system and method for measuring properties of a semiconductor substrate or of an electrical device formed on the substrate to provide rapid measurement of variations in the semiconductor fabrication process.
The increased complexity of semiconductor devices has lead to the need for measuring a number of different properties of the substrate that may affect the quality of the semiconductor substrate, such as the conductivity or the presence of defects. As the number of active devices, such as transistors, on a semiconductor substrate, for example, increases, there is a need to measure the various properties of different small areas (i.e., an area less than 2 square micrometers) of the substrate to a high degree of precision, especially as these areas are modified by the manufacturing process. This is especially the case when the substrate properties directly affect the characteristics of devices, such as transistors, diodes, or resistors, for example, formed on or contained within the semiconductor substrate.
There are a number of conventional methods and apparatus for measuring the properties of a semiconductor substrate, and for measuring the properties of active devices and passive devices, such as transistors, resistors, diodes, etc., formed on or in the substrate. For example, the properties of devices formed on or in the substrate may be measured directly once the device on the substrate has been completely manufactured by applying voltages to appropriate electrodes and measuring the response. The difficulty with measuring the properties after the active or passive device has been formed is that the long delay (several weeks) between early fabrication steps and completion makes tight process control impossible, resulting in excess variation in device parameters. There is also no way to change the device property parameter after the fabrication process, or vary the manufacturing process to improve the property of the completed device. In other conventional systems, the surface photovoltage of the substrate may be measured or resistance probes may be attached to the device or the substrate, and the property values inferred from indirect measurements. In these conventional systems, too much area on the substrate must be dedicated to the measurement apparatus for connecting the probes. Thus, these systems can not be used to control the fabrication line, or measure properties of devices during the manufacturing process. The property measurements are made in other systems by destroying the substrate, or the properties of a device are measured indirectly by measuring other properties and calculating the desired property.
None of these conventional methods and apparatus for measuring the properties of a substrate, or for measuring the properties of active or passive devices formed on the substrate, enable the actual property values to be measured during the actual fabrication process. In addition, known systems do not permit the measurement of the properties of very small areas of a substrate, such as a channel of a field effect transistor. These conventional methods for measuring properties are usually carried out on test substrates, and not actual production substrates, which reduces the likelihood that the method will detect localized problems within the manufacturing process. These known systems can not perform real-time monitoring and control of a semiconductor fabrication process.
Thus, there is a need for a system and method for real-time monitoring and measurement of properties in a semiconductor fabrication process to control the fabrication process which avoid these and other problems of known systems, and it is to this end that the present invention is directed.